It is okay for their belly to look flat. As you mention in your post, you do not want bones protruding. I usually gauge weight (visually) by looking at the tail and the limbs.
You do not mention the size of the enclosure and this is ‘extremely’ important.
Your basking temp and cool end temp is fine but keep in mind that a gradient temp does not include the basking spot. The gradient temp is a variance between ambient temps in the entire enclosure. I shoot for the majority of my enclosure to be between 90-100 with the largest portion of that being somewhere between 96-100. I shoot for temps between 80-85 on the cool end and prefer that this area be twice the size of a full-grown geryi. You want the majority of your ambient temps to be between 90-100 so that the uro does not have to bask all day under the basking spot. The proper gradient temp (in both actual temperatures and size of area) is difficult and sometimes impossible to achieve in a small enclosure. Everyone has their own preference when it comes to temperatures and proportions. I am just giving you an idea of mine for reference not as an absolute.
Have you tried chopped up frozen (and warmed) peas? Although not a staple food, peas can be a good way to get a new uro to eat. Once you have them eating, then you can worry about proper diet. Lentils work well too. I usually soak mine overnight prior to feeding as newly acquired uros sometimes tend to be a little dehydrated. Again, this should not be a staple food but can be a good enticer.
If the uro has not eaten for a while, dusting the food with a little beneficial bacteria once they are eating might be a good idea.
I am always concerned with vets (not all) and their mission to eradicate every parasite in sight. If your vet does a fecal, (which I would recommend) and wants to medicate- ask questions. What exactly is he/she seeing, in what amounts is he/she seeing it, and how necessary is it to medicate immediately. Although controversial, ‘I’ do not believe in always eradicating everything. I also do not believe in medicating a newly shipped uro unless it is necessary. Too many times newly acquired stressed out Saharans are not eating solely because they are stressed. They are then taken to a vet and the vet discovers a few minor parasites so the uro is put on medication. The already stressed out uro now has the added stress of being medicated and the loss of natural gut flora so the uro still does not eat. Sometimes the vet decides that the medication is not working so a new medication is prescribed. I have heard too many horror stories like this only to have the story end the same- in the death of the uromastyx. In my opinion, some of these uros (especially geryi) need a little time to acclimate before they are bombarded with medication and additional stress. There are definitely times when a newly acquired uro should be medicated and I believe that there are times when medication could wait; it all depends on the results. Again, this is controversial and only ‘my’ opinion. My point here I guess is- ask questions. (or print this paragraph off and give it to your vet) 